Waste gas heat exchanger

ABSTRACT

The invention relates to a heat exchanger, especially a heat exchanger for motor vehicles, comprising a bank of tubes through which a gaseous medium flows and around which a liquid coolant flows. The ends of said tubes are received in tube plates and are connected to the same in a material fit. The inventive heat exchanger also comprises a housing jacket which surrounds the bank of tubes and is connected, at the end thereof, to the tube plates. A coolant flows through said housing jacket. The tubes ( 5 ), tube plates ( 3, 7 ) and housing jacket ( 2 ) are produced from a heat-resistant and corrosion-resistant metallic alloy. According to the invention, the housing jacket ( 2 ) comprises at least one surrounding expansion flange ( 4 ).

[0001] The invention is concerned with a heat exchanger, in particularan exhaust gas heat exchanger for motor vehicles, in accordance with theprecharacterizing clause of patent claim 1, as disclosed by DE-A 199 07163 of the co-applicant. The invention is furthermore concerned with amethod for producing a housing jacket of an exhaust gas heat exchangerin accordance with the precharacterizing clause of patent claim 12.

[0002] In the case of the exhaust gas heat exchanger disclosed by DE-A199 07 163, a bank of tubes has the exhaust gas of an internalcombustion engine of a motor vehicle flowing through it and is cooled onthe outside by a coolant which is taken from the coolant circuit of theinternal combustion engine. Exhaust gas heat exchangers of this type,which are also called exhaust gas coolers, are used nowadays in theexhaust gas recirculation system (EGR) to cool the exhaust gas. In thecase of the known exhaust gas cooler, the tube ends of the bank of tubesare welded in each case into a tube plate, i.e. are connected fixedlyand tightly to these tube plates. The tube plates themselves are weldedin turn to a housing jacket which surrounds the bank of tubes. Thehousing jacket has a coolant inlet opening and a coolant outlet openingand has the coolant flowing through it. During operation of an exhaustgas cooler of this type, the exhaust gas tubes have the hot exhaust gasflowing through them on the inside and have coolant washing around themon the outside. This coolant also washes around the inside of thehousing jacket. The exhaust gas tubes therefore reach a substantiallyhigher temperature than the housing jacket, which results in differentexpansions between the exhaust gas tubes and housing jacket: this leadsto thermal stresses, i.e. to compressive stresses in the tubes andtensile stresses in the housing jacket. The tubes press on the tubeplates and cause deformation or even damage to the tube/plateconnections or the tube plate/housing connections, i.e. the exhaust gascooler can become leaky.

[0003] In similar exhaust gas coolers in EP-A 0 930 429 a “sliding fit”has therefore already been proposed, i.e. the bank of tubes is arrangedin the housing of the exhaust gas cooler by means of a fixed bearing anda movable bearing, i.e. the tubes can expand unimpeded owing to the tubeplate being mounted in a sliding manner in the housing. Although thermalstresses are avoided as a result, an increased structural outlay isrequired for a sliding fit of this type; in addition, there is the riskthat if the sliding fit is insufficiently sealed, coolant will pass intothe exhaust gas or exhaust gas will pass into the coolant.

[0004] It is therefore the object of the present invention to improve aheat exchanger of the type mentioned at the beginning to the effect thatthe stresses caused by temperature are compensated for by simplemeasures, i.e. impermissible loads on the material are avoided.

[0005] This object is achieved for the heat exchanger according to thegeneric type by the characterizing features of patent claim 1, i.e. thehousing jacket is provided with at least one encircling expansion bead.This bead provides the housing jacket with sufficient elasticity in thelongitudinal direction of the tubes, thus making it possible for thehousing jacket to expand elastically so as to follow the more pronouncedexpansion of the exhaust gas tubes without in the process being deformedto an impermissible extent or impairing the weld seam connectionsbetween the tubes and plate and plate and housing. In addition, theexpansion bead can be produced in a simple manner, i.e. withoutsubstantially greater costs, and does not involve any sort of sealingproblems. It is also possible—to increase the elasticity or to enlargethe spring deflection—to provide a plurality of extension bead in themanner of an expansion bellows.

[0006] According to one advantageous refinement of the invention, thehousing jacket is produced integrally, for example from a welded tube,it also being possible for said tube to have a noncircular crosssection, for example a rectangular cross section.

[0007] According to a further advantageous refinement of the invention,the expansion bead is produced by “internal high pressure forming” (IHF)of the housing jacket. The IHF, which is also called hydroforming, is aprocess which is known per se and in which closed housing parts are“inflated” by means of a liquid pressure medium (water). The housingswhich are to be deformed are placed into dies having the appropriatecontour and are then acted upon from the inside by means of a pressurefluid in such a manner that the material of the housing is placedagainst the contour of the mold.

[0008] According to a further advantageous refinement of the invention,the expansion bead can additionally be produced by axial compression,i.e. after a bead in preliminary form has been produced in a first stepby IHF.

[0009] In further advantageous refinements of the invention, dimensionsfor the housing jacket, in particular the wall thickness thereof, andthe dimension of the expansion bead are specified, said dimensions beingparticularly advantageous and resulting in the desired elasticity of thehousing jacket under the loads which occur. In this case, it is alsoensured that the material of the housing jacket does not over-expandduring production of the expansion bead, but that the designatedstrength is achieved.

[0010] Finally, one advantageous refinement of the invention provides amethod which enables simple and cost-effective production of theexpansion bead in the housing jacket of the exhaust gas heat exchanger.According to this method, the expansion bead is produced in two stages,namely first of all by means of internal high pressure forming to give abead which is in a preliminary form and is not yet in the final form, inparticular does not yet have the final height (external dimensions). Ina second method step, the housing jacket is compressed axially, thuscausing the material of the bead in preliminary form to flow furtheroutward, and the expansion bead then obtains its final form. Thistwo-stage method avoids overloading the material and, at the same time,achieves a defined contour of the expansion bead with a certainelasticity. This method can be used particularly easily for the housingjacket and does not cause any change in the construction of the exhaustgas heat exchanger.

[0011] An exemplary embodiment of the invention is illustrated in thedrawing and will be described in greater detail below. In the drawing:

[0012]FIG. 1 shows a perspective illustration of part of the exhaust gasheat exchanger,

[0013]FIG. 2 shows a longitudinal section through the exhaust gas heatexchanger,

[0014]FIG. 3 shows a cross section through the housing jacket of theexhaust gas heat exchanger,

[0015]FIG. 4 shows an illustration of a detail of the expansion bead ofthe housing jacket,

[0016]FIG. 5 shows a first method step for producing the expansion bead,and

[0017]FIG. 6 shows a second method step for producing the expansionbead.

[0018]FIG. 1 shows, in a perspective illustration, part of an exhaustgas heat exchanger 1 as used in the form of an exhaust gas cooler forthe exhaust gas recirculation system in diesel engines for motorvehicles. The exhaust gas heat exchanger 1, only the front half of whichis illustrated, has a housing jacket 2 and a tube plate 3 in whichexhaust gas tubes (not illustrated) are accommodated. An encirclingexpansion bead 4 is arranged in the front region of the housing jacket2, which has approximately a rectangular cross section with beveledcorners. This exhaust gas heat exchanger 1 is described in greaterdetail below, the same reference numbers being used for the same parts.

[0019]FIG. 2 shows the exhaust gas heat exchanger 1 in longitudinalsection in a schematic illustration. The housing jacket 2 is producedfrom a closed, i.e. welded tube of stainless steel. A bank of tubescomprising a multiplicity of exhaust gas tubes 5 is arranged within thishousing jacket 2. These exhaust gas tubes 5 are likewise produced from astainless steel alloy which is, in particular, heat-resistant andcorrosion-resistant. The cross section of the exhaust gas tubes 5 ispreferably rectangular—as is apparent on the basis of the design of thetube plate 3 in FIG. 1. The exhaust gas tubes 5 are arranged withrespect to one another in such a manner that they leave between themequidistant gaps 6 through which a liquid coolant, i.e. the coolant ofthe cooling circuit of an internal combustion engine (not illustrated),flows. The ends 5 a, 5 b of the exhaust gas tubes 5 are held in the tubeplate 3 and in a further tube plate 7 and are welded tightly to thesetube plates 3, 7. The tube plates 3, 7 are, for their part, welded intheir circumferential regions 3 a, 7 a to the housing jacket 2 to thehousing jacket 2. The housing jacket 2, the tube plates 3, 7 and theexhaust gas tubes 5 therefore delimit a defined space for the flow ofthe coolant. In its end-side regions 2 a and 2 b, the housing jacket 2is somewhat expanded in terms of its cross section, so that it forms arespective annular channel 8 and 9 around the bank of tubes at thispoint. In this region, the coolant enters through a coolant inlet (notillustrated), flows through the equidistant gap 6 between the exhaustgas tubes 5, and passes to the second annular space 9, from where thecoolant leaves the exhaust gas heat exchanger through a coolant outlet(not illustrated). To this extent, this exhaust gas heat exchanger isknown, for example through the co-applicant's document which ismentioned at the beginning. The inflow and outflow of the exhaust gasvia a diffuser (not illustrated here) or an outlet stub is also revealedin this document.

[0020] According to the invention, an expansion bead 4 is arranged inthe housing jacket 2. This expansion bead 4, which can also be seen inFIG. 1 as the bead which encircles the entire circumference, gives thehousing jacket 2, which is not very elastic in itself, an elasticity inthe longitudinal direction of the exhaust gas tubes 5, said elasticitypermitting the housing jacket 2 to follow the more pronounced expansionof the exhaust gas tubes 5. The exhaust gas tubes 5, which have hotexhaust gas flowing through them on their inside, absorb a highertemperature during operation than the housing jacket 2, around which thecoolant washes, and therefore “grow” to a greater extent than thehousing jacket. Compressive stresses are therefore produced in theexhaust gas tubes 5, these stresses continuing into the tube plates 3and 7 and being transmitted by the latter via the weld seams to thehousing jacket 2, in which a tensile stress then builds up. This tensilestress is intercepted owing to the elasticity of the expansion bead 4,so that impermissible deformation or even damage does not occur.

[0021]FIG. 3 shows a cross section through the housing jacket 2, i.e.without the bank of tubes being illustrated. The housing jacket 2 has anapproximately rectangular cross section with in each case two parallellonger side surfaces 10 and 11 and two somewhat shorter side surfaces 12and 13 lying opposite each other. Longitudinal beads 14, 15, 16, 17 forstiffening the entire cross section are formed in the transition regionsof adjacent short and long side surfaces 12/10, 10/13, 13/11 and 11/12.One of these beads can also be seen clearly in FIG. 1—denoted by 14there. The expansion bead 4 is situated somewhat offset to the rear ofthe plane of projection and surrounds the entire cross section of thehousing jacket 2, i.e. it is of encircling design.

[0022] The cross section and the dimensions of this expansion bead 4 areillustrated in FIG. 4. The wall thickness of the housing is denoted by sand is s≈1.5 mm. The bead 4 has a width of b≈6 mm and a height of h≈6mm. The bead is furthermore characterized by two transition radii R1 andR2 which correspond approximately to the wall thickness s, i.e. lie inthe region of 1.5 mm. The outermost section of the bead is characterizedby an inner radius of R3≈1.5 mm, i.e. approximately of the wallthickness S. These radii ensure that no impermissible expansions orstress peaks occur either during production or during operation.

[0023]FIG. 5 shows a first method step for producing the expansion bead4 in the housing jacket 2. The housing jacket 2 is illustrated here onlyby a sector 20. The housing jacket 2 is placed into two mold halves 21and 22, between which a cavity 23 is provided which is preferably closedto the outside by an insertable tool 23′. The housing jacket 20 is actedupon from the inside, illustrated by an arrow p, by hydroforming or byIHF, so that the housing jacket 20 is deformed in the region of thecavity 23 to the outside to height h1 and takes on a bead-shapedpreliminary form 24. This bead 24 in preliminary form has a width b1corresponding to the cavity 23 between the two mold halves 21 and 22.

[0024]FIG. 6 shows the second method step for producing the expansionbead 4—in this case the housing jacket 20 is arranged between twoaxially movable molds 25 and 26. After the hydroforming according toFIG. 5, the housing jacket 20 is compressed by means of the molds 25 and26 in the axial direction, i.e. corresponding to the arrows F, so thatthe width of the bead is reduced from b1 (FIG. 5) to b and the height h1(FIG. 5) is increased to h. After this method step of axial compression,the bead has obtained its final form in respect of height and width,i.e. it is finished in two consecutive different method steps.

[0025] The two method steps can be carried out in one mold, in whichcase the insertable tool 23′, if the operation is carried out using aninsertable tool of this type, has to be removed for the second methodstep. However, it is also possible to carry out the two method steps ina number of molds or in one follow-on mold.

1. A heat exchanger, in particular an exhaust gas heat exchanger formotor vehicles, having a bank of tubes through which a gaseous mediumflows and around which a liquid coolant flows, and the tubes of whichare held by their tube ends in tube plates and are connected theretowith a cohesive material joint, and having a housing jacket whichsurrounds the bank of tubes and is connected on the end side to the tubeplates with a cohesive material joint and through which the coolantflows, tubes (5), tube plates (3, 7) and housing jacket (2) beingproduced from a heat-resistant and corrosion-resistant metallic alloy,characterized in that the housing jacket (2) has at least one encirclingexpansion bead (4).
 2. The heat exchanger as claimed in claim 1,characterized in that the housing jacket (2) is of integral design. 3.The heat exchanger as claimed in claim 1 or 2, characterized in that thehousing jacket (2) is produced from a welded tube.
 4. The heat exchangeras claimed in claim 1, 2 or 3, characterized in that the housing jackethas a noncircular cross section (10, 11, 12, 13).
 5. The heat exchangeras claimed in one of the preceding claims, characterized in that theexpansion bead (4) is produced by internal high pressure forming (IHF)of the housing jacket (2).
 6. The heat exchanger as claimed in one ofthe preceding claims, characterized in that the expansion bead (4) isproduced by axial compression of the housing jacket (2).
 7. The heatexchanger as claimed in one of the preceding claims, characterized inthat the housing jacket (2) has a wall thickness of 0.5≦s≦2.5 mm,preferably of s≈1.5 mm.
 8. The heat exchanger as claimed in claim 7,characterized in that the expansion bead (4) has a height h of 2≦h≦10mm, in particular of h≈6 mm.
 9. The heat exchanger as claimed in claim 7or 8, characterized in that the expansion bead (4) has a width b of4≦b≦8 mm, in particular of b≈6 mm.
 10. The heat exchanger as claimed inone of claims 7, 8 or 9, characterized in that the ratio of width toheight, i.e. b: h≈1.
 11. The heat exchanger as claimed in one of claims7 to 10, characterized in that the bead (4) has a bending radius ofR3≈s.
 12. A method for producing an expansion bead (4) in a tubularhousing jacket (2, 20), in particular having a noncircular crosssection, characterized by the following method steps: provision of ahousing jacket (2, 20) which is cut to size, insertion of the housingjacket (2, 20) into an IHF mold (21, 22, 23) and closing the mold,filling the mold and the housing jacket (2, 20) with a liquid pressuremedium, deformation of the housing jacket (2, 20) by building upinternal high pressure and producing a preliminary form (24) of the bead(first deformation step), reduction of the internal high pressure andproduction of the final form of the expansion bead (4) by axialcompression of the housing jacket (2, 20) in a second deformation step.13. The method as claimed in claim 12, characterized in that the firstmethod step production of a preliminary form (24) of the bead, and thesecond method step axial compression of the housing jacket (2, 20) arecarried out in a mold.